In ladder diagrams, what do rungs connect?

• A. Ground lines to power
• B. Power supply to the control relay
• C. The rails to the logic expressions
• D. The input devices to the PLC

Answer: (C)

Rungs are the horizontal paths that bridge the two power rails with the logic elements arranged on them. They carry the logic expression—the series and parallel arrangement of contacts and coils—that determines whether a complete current path exists from the left power rail to the right return. When the conditions represented by those contacts are met, the path is completed and the output coil or device on that rung is energized. So the rung’s job is to connect the rails through the logic expressions that control the output.

For example, a rung may have two contacts in series representing two conditions both being true. If both are true, current can flow from the left rail, through the contacts, to the right rail, energizing the coil on that rung.